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SAG
S-antigen visual arrestin
SAG is located on the long (q) arm of chromosome 2, at band 2q37.1. Arm ratio per GRCh38 - banding schematic.
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Overview
SAG (S-antigen visual arrestin) is located on chromosome 2 at position 2q37.1 and encodes a 405-amino acid protein known as arrestin or S-antigen. This protein plays a critical role in the phototransduction cascade, the biochemical process by which light is converted into electrical signals in the retina. When light strikes photoreceptor cells, arrestin binds to activated rhodopsin, halting the visual signal and allowing the cell to reset for subsequent stimulation. Pathogenic changes in SAG disrupt this regulatory mechanism, contributing to progressive retinal degeneration. The gene is recognised in UK clinical practice for its association with inherited retinal conditions.
What the gene does
Arrestin functions as a molecular switch in photoreceptor cells, both rods and cones. Following light exposure, rhodopsin undergoes a conformational change and activates the G-protein transducin, initiating a signalling cascade that ultimately generates an electrical impulse. Arrestin binds specifically to light-activated, phosphorylated rhodopsin, preventing further activation of transducin and effectively terminating the signal. This process, known as desensitisation, is essential for photoreceptor recovery and adaptation to varying light intensities. Without proper arrestin function, photoreceptors remain in a prolonged activated state, leading to cellular stress, dysfunction, and eventual degeneration. Arrestin also participates in the recycling of rhodopsin back to its inactive state, supporting the continuous regeneration of visual pigment required for normal vision. The protein's regulatory role extends beyond simple signal termination, influencing the overall health and longevity of photoreceptor cells.
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Chromosome location
The SAG gene is positioned on the long arm of chromosome 2 at band 2q37.1. This chromosomal region contains several genes involved in sensory and neurological functions. The precise genomic architecture and exon-intron structure of SAG contribute to the diversity of transcripts and protein isoforms, although detailed structural annotations are not universally standardised across all databases.
Protein structure
Domain architecture has not been experimentally characterised in detail for this protein.
Key variants
Pathogenic variants in SAG typically result in reduced or absent arrestin protein, or in some cases, a dysfunctional protein that cannot properly bind to activated rhodopsin. The spectrum of variants includes missense changes that alter critical binding sites, nonsense mutations leading to truncated protein, and splice-site alterations affecting transcript stability. The functional impact of a variant often correlates with disease severity, although phenotypic variability is observed even among individuals carrying the same genetic change.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Variants in SAG are associated with inherited retinal disorders characterised by progressive vision loss. These conditions typically present with symptoms such as night blindness, reduced visual acuity, and constriction of the visual field. The age of onset and rate of progression vary depending on the specific genetic alteration and individual factors. Because retinal degeneration can significantly impact quality of life, early identification of SAG variants through genetic testing supports informed management and access to specialist ophthalmology care.
No disease links recorded for this gene in our reference set.
UK clinical status
SAG is included on the NHS Genomic Medicine Service Retinal Disorders panel (green classification, version R32). This designation indicates that the gene has sufficient evidence linking it to inherited retinal conditions and is approved for diagnostic testing within the NHS. Individuals with relevant clinical features may be referred for panel testing, which examines SAG alongside other retinal disorder genes to identify a molecular diagnosis.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What does the SAG gene do?
SAG encodes arrestin, a protein that terminates light-activated signals in photoreceptor cells of the retina. By binding to activated rhodopsin, arrestin prevents prolonged signalling and allows the visual system to reset, which is essential for normal vision and photoreceptor health.
How are SAG variants inherited?
SAG-related conditions can follow different inheritance patterns depending on the specific variant and associated disorder. Some variants act in a recessive manner, requiring changes in both gene copies, while others may contribute to disease through different mechanisms. Genetic counselling can clarify the inheritance pattern relevant to an individual or family.
Is genetic testing for SAG available on the NHS?
Yes, SAG is included on the NHS Retinal Disorders panel. Individuals with clinical signs of inherited retinal disease may be referred by their ophthalmologist or clinical geneticist for panel testing, which examines SAG and other relevant genes to support diagnosis and management.